If you don't have an option to do that in your customization options of your theme, either in the general theme settings or the product page settings, then you will need to make this change in your theme.scss.liquid file. You will need to find the font-style definition which is being used by your product descriptions and change the "font-size" property of it. Alternatively, you could add a setting to your product customization options to allow you to change this easily whenever you want to.

If you would like further help with this, please feel free to send me an email at support@microDev.today and we can help you achieve this or any other customizations you would like to do.